All things being equal, bigger is safer. I believe the NHSTA site had also mentioned this in the disclaimer on the car test results. But then how big are you gonna go? Get an 18 wheeler?
So I think you should get something that meets her needs and she likes and call it a day.
The driver is more important. My wife has been driving for a few years but for my newer driver daughter I was a bit worried. So, we didn’t go with a hand me down beater with no safety features, neither did we get the tinniest car on the market. We went with something in the middle. Anything larger would had been difficult for her to handle, wiping all the size related safety.